1. Field of the Invention
This invention relates to a method for producing a tin coating that resists oxidizing. More particularly, this invention relates to a method for introducing selected additions of a material into a tin coating that reduce discoloration of the tin coating.
2. Background of the Invention
Tin coatings are frequently applied to copper alloy devices such as leadframes and electrical connectors. One function of the tin coating is to prevent copper alloy surfaces from oxidizing or tarnishing. A tarnish-free surface has lower electrical contact resistance than an oxide coated surface and also has better solderability.
Under conditions such as elevated temperatures in air or other oxygen containing atmospheres, tin coatings have a tendency to oxidize, producing oxide films that discolor the surface of the tin coatings with a yellowish color. Although the oxide film is typically only about 50-200 Angstroms in thickness, the surface of the tin may turn a yellow color, which many consumers consider unacceptable.
Under some conditions, such as elevated temperature environments, the oxidized growth may attain a thickness that degrades the contact resistance of a coated electrical terminal.
There have been attempts to address some of these shortcomings. These prior attempts failed to provide an efficient way to prevent oxide growth on a tin coating.
A publication entitled, "An Examination of Oxide Films on Tin and Tin Plate", by S. C. Britton and K. Bright discloses that the addition of small amounts of phosphorous, indium or zinc to tin prevents the formation of color films when the metal is heated. Although this article recognizes the need to prevent oxidation of tin, it does not disclose an efficient method to introduce oxide resisting elements into the tin coating such that these elements have an increased concentration at the surface of the tin that is exposed to ambient air.
Japanese Kokai No. 3(1991)-239,353 published Oct. 24, 1991, discloses a copper leadframe for semiconductor devices. This reference describes placing zinc between a copper leadframe and a tin coating. The zinc layer is introduced to prevent diffusion between the tin coating and the copper leadframe. This reference also fails to disclose an efficient method for applying selected oxide resistant elements into a surface of a tin layer exposed to ambient air.
As can be seen from the illustrative background discussed above, there is a need for a method to provide an anti-tarnish, oxide resistant, agent such as indium, phosphorous or zinc into tin coatings in order to provide protection against yellowing in elevated temperature environments, such as encountered by electrical terminals connected to sources of high voltage and current and automotive connectors. The present invention provides a solution to that need in the form of a procedure to add material to tin coatings to form a composite coating of tin and an anti-tarnish agent that resists oxidation.